Why are english reins so long?

It's for various different reasons.

The length of the horse's neck obviously.

It allows for varying contacts and also so that you can allow the horse to stretch down on a long rein. Allowing the horse to stretch before and after each session is important. Also in dressage one aspect of the test is a free walk - drop the contact almost down to the buckle and let the horse stretch right down.

When jumping, especially on the landing you need to allow the horse their head so that that can find their balance and it can often save both horse and rider if an error is made. If you have little or no rein with larger fences or a drop then you won't be able to do this.

A good example is to watch Eventing or Puissance jumping, look at the landing. The rider's quite commonly allow the horse plenty of rein so they can recover after the fence.

They are made long because some horses have VERY long necks and need the length. With western reins there split so it doesn't matter the length because you can adust it. Some riders do ride their horse on the buckle and use the whole reins. Its just exsess in case of an emergency or something.

Their necks are so much longer and the longer reins can be used a lot easier to pull in their noses and make the horse look a little 'prettier' it also depends on like whether your riding them Hunt Seat or Saddle Seat. If you ride them in Saddleseat it makes it a lot easier to shorten your reins and get their head into the right position.